This invention relates generally to organic rankine cycle systems and, more particularly, to the use of an improved working fluid in such systems.
The well known closed rankine cycle comprises a boiler or evaporator for the evaporation of a motive fluid, a turbine fed with vapor from the boiler to drive the generator or other load, a condenser for condensing the exhaust vapors from the turbine and a means, such as a pump, for recycling the condensed fluid to the boiler. Such a system as is shown and described in U.S. Pat. No. 3,393,515.
Such rankine cycle systems are commonly used for the purpose of generating electrical power that is provided to a power distribution system, or grid, for residential and commercial use across the country. The motive fluid used in such systems is often water, with the turbine then being driven by steam. The source of heat to the boiler can be of any form of fossil fuel, e.g. oil, coal, natural gas or nuclear power. The turbines in such systems are designed to operate at relatively high pressures and high temperatures and are relatively expensive in their manufacture and use.
The Organic Rankine Cycle (ORC) is a vapor power cycle with refrigerant (an organic fluid) instead of water/steam as the working fluid. Functionally it resembles the steam cycle power plant: a pump increases the pressure of condensed liquid refrigerant. This liquid is vaporized in an evaporator/boiler by extracting waste heat from turbine or engine exhaust. The high-pressure refrigerant vapor expands in a turbine, producing power. The low-pressure vapor leaving the turbine is condensed before being sent back to the pump to restart the cycle.
For refrigerants with certain properties, commercially available air-conditioning and refrigeration equipment with its proven reliability and performance record can be used cost effectively in a power producing ORC system.
The rankine cycle used for power generation production goes through the following four processes in this order:                1. Adiabatic pressure rise through a pump        2. Isobaric heat addition in a preheater, evaporator and superheater        3. Adiabatic expansion in a turbine        4. Isobaric heat rejection in a condenser.        
The main thermodynamic irreversibility in organic rankine cycles is caused by the large temperature difference in the evaporator between the temperature of the waste heat stream and the boiling refrigerant. The higher the waste heat stream temperature is the larger this irreversibility becomes. Organic fluids that can boil at higher temperatures have the ability to reduce the temperature difference between the waste heat stream and the organic rankine cycle working fluid and will therefore result in higher thermodynamic ORC cycle efficiency. Fluids can boil at temperatures up to the critical temperature, above which there is no boiling. Consequently, fluids with higher critical temperatures will result in higher ORC cycle efficiency. Chlorine containing fluids with high critical temperatures have been proposed in the past as ORC fluids. For example, R114, R113, R11, R141b and R123 have higher critical temperatures than R245fa and would result in substantially higher thermal efficiencies. However, these fluids are either flammable and/or toxic, ozone layer depleting and/or have substantial global warming impact. They have either been banned (the CFC's) or soon will be banned (the HCFC's) and therefore will not be available for use in future ORC products.